1. Field of the Invention
The present invention relates to a retro-reflectivity array, and more particularly, to a retro-reflectivity array comprising a camera with a high speed sensor which can perform pupil tracking.
2. Description of the Prior Art
Retro reflectivity is a property of the human eye, meaning that when a light source is shone into the eye, a majority of the incident light will be reflected back in the direction from which it came. This property gives rise to what is commonly known as the ‘red-eye effect’ in photographs. An optical axis can be imagined as a line running between the centre of an imager, such as a camera, and the centre of the pupil of a human eye. In order for the eye's retro-reflective properties to be detected, the light source shining into the eye should be located next to the imager (on-axis) so that the pupil reflects the light back towards the source. Then it will show up as a bright spot. When the light source is located away from the imager (off-axis), the retro-reflection from the pupil will miss the imager and the pupil will appear dark in the image.
By using this red-eye effect, both the presence and location of a person's eyes can be determined. Please refer to FIG. 1, which is a diagram illustrating a system 100 for performing pupil detection according to the related art. A human eye 50 is positioned opposite an imager 30. LED arrays 15, 13 are placed either side of the imager 30. The LED array 15 comprises an on-axis LED 2 and an off-axis LED 7; the LED array 13 comprises an on-axis LED 4 and an off-axis LED 9. Certain calibration procedures will be performed to determine the on-axis illumination angle and the off-axis illumination angle, wherein the on-axis illumination angle can be calibrated by the imager 30 detecting reflected light from the pupils above a certain threshold, and the off-axis illumination angle can be calibrated by the imager 30 capturing an image with completely dark pupils. By differencing the two images, only the pupil information will remain. It is therefore possible to determine for what percentage of time the user's eyes are open or visible to the imager (e.g. the person has not turned around). Retro-reflection will be viewed from the pupil regardless of gaze angle in any configuration where the LED is near the imager and the pupil is visible.
One application of this technology is detailed in U.S. Pat. No. 7,280,678. A retro-reflectivity array similar to that illustrated in FIG. 1 is used to detect when a driver of a car is falling asleep. Because the driver should be looking ahead at the road, the driver's gaze angle should be relatively fixed. By setting a threshold which takes into account occasional glances over the shoulder as well as blinking, the pupils should be able to be detected for a certain percentage of time above that threshold value. When the pupil detection falls below the threshold value, this indicates that the driver is falling asleep (or distracted) and an alarm or similar warning system can be activated.
This technology is limited by the speed of a conventional sensor and processor used in such applications, which allow image frames to be caught and processed at less than 60 frames per second. At these speeds, it is possible to determine the presence of pupils, i.e. whether or not a user is alert. However, if the subject is moving quickly, both the on-axis and off-axis images must be collected simultaneously using specialized optical filters. Although the differenced images of the concept described above are able to successfully detect pupils, the present invention aims to explore an alternative method of implementation using an image-based approach with a high-speed camera to detect pupils. In this case, the frame rate is so high that the on-axis and off-axis frames can be collected one after the other without requiring specialized optical filters.